U.S. Pat. No. 5,742,463 proposes various active circuits which may function as a protection device and serve as an alternative to known fuses for inhibiting the transmission of transients to a load.
The practical embodiments disclosed in U.S. Pat. No. 5,742,463 employ depletion mode junction field effect transistors (JFETS) and utilize commutating diodes in those embodiments where the number of components has been minimised. U.S. Pat. No. 5,742,463 suggests that depletion mode metal oxide semi conductor field effect transistors (MOSFETS) may be used as alternatives to junction field effect transistors (JFETS).
The operation of the embodiment of U.S. Pat. No. 5,742,463 is such that once the JFETS that have their conduction paths in series are in the cut-off phase of their conduction characteristics in response to the present of a transient of a sufficient magnitude, those JFETS cannot return to their original conducting phase until the applied input returns to zero or near zero.
Thus, the embodiments of U.S. Pat. No. 5,742,463 were unsuitable for operation with inputs which were level shifted and where transients occurred and the input did not return to zero or near zero after the occurrence of the transient that caused the series connected transistors to assume their cut-off condition. In addition, U.S. Pat. No. 5,742,463 required the presence of commutating diodes.